PNNL

Abstract

Combined scanning tunneling microscopy (STM), x-rays photoelectron spectroscopy (XPS) and density functional theory (DFT) studies have probed the bonding configurations and mobility of trimethylacetic acid (TMAA) molecules on TiO2(100) surface at RT. Upon TMAA dissociation through deprotonation, two distinctly different types of stable chemisorption configurations of the carboxylate group (TMA) have been identified accordingly to their position and appearance in STM images. In configuration A, two carboxylate O atoms bonds to two Ti4+ cations, while in configuration B one O atom fills the bridging oxygen vacancy (VO) with the other O bounded at an adjacent regular Ti4+ site. Calculated adsorption energies for the configurations A and B are comparable at 1.28 and 1.36 eV, respectively. DFT results also show that TMA may rotate at RT about its O atom that filled the VO (in configuration B), with a rotation barrier of ~0.65 eV. Both the observation of the constant initial sticking coefficient and preference for TMAA molecules to dissociate at selective sites indicate that TMAA adsorption is mediated by a mobile precursor state. Several possible molecular (physisorbed) states of TMAA have indeed been identified by DFT, all being a highly mobile at RT. In contrast, the TMA diffusion in chemisorbed (dissociative) state is a very slow with calculated barrier of 1.09 eV for diffusion along the Ti row.